The information amassed through the lifetime of individual fishermen by natural observation is of a scale and quality not normally accessible to scientific surveys. Obtaining such information and rendering it scientifically useful is one step in gaining new insights in the functioning of marine ecosystems and fisheries. In this paper, we illustrate this idea by analysing the rose shrimp (Aristeus antennatus) fishery in the Catalan Sea (NW Mediterranean). Catalan fishermen assert that the catch of rose shrimp in the area is higher on Fridays than on other days of the week. We tested the validity of this assertion by analysing a 2-year data set on landings and sale prices for rose shrimp at the port of Barcelona (Spain). We established three hypotheses to account for the fact that rose shrimp catch is highest on Fridays. The first hypothesis assumes that as trawling proceeds during the week, sediment resuspension is increased over the fishing grounds and food opportunities for the rose shrimp are enhanced and reach its maximum on the last day of the working week (Fridays). The second hypothesis states that competitors (mainly demersal fish) are removed from the fishing grounds at a higher rate than shrimp, thus fish catch would decrease towards the end of the week, while shrimp catch would increase. The third hypothesis is based on the assumption that market dynamics determine the higher shrimp catch on Fridays by the higher prices fetched by this species at the auction, thus driving fishermen to concentrate on rose shrimp towards the end of the week. Analysis of the data in the light of the three hypotheses indicates that only hypothesis two and three could have a real influence. We establish the working mechanisms to account for this result: the ability of fishermen to remove competitors at a differentially higher rate and the better location of rose shrimp shoals in the course of the week, leads to a higher catch of rose shrimp in the study area towards the end of the week. The subconscious factors at work during the fishing process coupled with the ecological behaviour of the species targeted result in the small-scale temporal variability in shrimp catch observed by Catalan fishermen

9 pages, 4 figures, 4 tables